单选题 A group of scientists studying calcium metabolism in laboratory rats discovered that removing the rats' parathyroid glands resulted in the rats' having substantially lower than normal levels of calcium in their blood. This discovery led the scientists to hypothesize that the function of the parathyroid gland is to regulate the level of calcium in the blood by raising that level when if falls below the normal range. In a further experiment, the scientists removed not only the parathyroid gland but also the adrenal gland from rats. They made the surprising discovery that the level of calcium in the rats' blood decreased much less sharply than when the parathyroid gland alone was removed.
Which one of the following, if true, explains the surprising discovery in a way most consistent with the scientists' hypothesis?

A、 The adrenal gland acts to lower the level of calcium in the blood.
B、 The adrenal gland and the parathyroid gland play the same role in regulating calcium blood levels.
C、 The absence of a parathyroid gland causes the adrenal gland to increase the level of calcium in the blood.
D、 If the adrenal gland, and no other gland, of a rat were removed, the rat's calcium level would remain stable.
E、 (E) The only function of the parathyroid gland is to regulate the level of calcium in the blood.

【正确答案】 A

【答案解析】[解析] 本题的两个结论是：(1)去除甲状旁腺时老鼠血液中的钙的水平比正常水平低得多；(2)同时去除甲状旁腺和肾上腺时，老鼠血液内钙的水平的下降比单是去除甲状旁腺时慢得多。对这两个结论进行比较，我们不难发现肾上腺一定具有促使老鼠的血液中钙的水平降低的作用。因此(A)，肾上腺的作用是降低血液中的钙的水平是正确的；而(B)、(C)和(D)都是不正确的；从短文中我们只能推出调节血液中的钙的水平只是甲状旁腺的一个功能，因此(E)也是不正确的。